1. Field of the Invention
This invention relates generally to polarimeter systems for measuring polarization properties of light and more particularly to an ophthalmological system for measuring the birefringence of structural elements in the eye.
2. Description of the Related Art
The polarimeter is well-known in the optical arts and is reviewed here briefly to establish some of the terminology required for this disclosure. A single-beam polarimeter measurement usually consists of an optical signal in a single state of polarization. Some form of xe2x80x9canalyzerxe2x80x9d within the polarimeter removes all but a single state of polarization from the incoming light, which is then measured and recorded with a suitable detector as a charge-coupled device (CCD). A series of measurements is usually made with a different state of polarization being recorded for each. These measurements allow both the degree and orientation of the optical signal polarization to be estimated and recorded. The single rotatable analyzer passes only light polarized parallel to a specified axis so the analyzer must be rotated about the optical beam axis to measure light polarized in different directions. A single fixed analyzer passes light polarized parallel to its axis and cannot be rotated but a polarization rotator such as, for example, a half-wave plate, may be placed in the optical beam axis to rotate the plane of polarization of the incoming optical signal before it reaches the fixed analyzer. Light polarized in different directions can thus be measured by rotating the half-wave plate.
A half-wave plate has a preferred (xe2x80x9cfastxe2x80x9d) axis. Light polarized parallel to this axis passes through the half-wave plate unchanged. Light polarized perpendicular to the fast axis (parallel to the xe2x80x9cslowxe2x80x9d axis) is retarded by half a wavelength. The net effect of this is to rotate the plane of polarization of the light so that the axis of the half-wave plate bisects the angle between the planes of polarization in the incoming and outgoing light. Using similar reasoning, it may be shown that the net effect of a precise one-quarter wavelength retardance is to bias the linear polarization components of the entering light into equivalent circular polarization components, as is well-known in the art.
The single beam polarimeter is exemplified by the polarimeter 20 shown functionally in FIG. 1. The optical signal 22 arrives along the optical beam axis 24 and the half-wave plate 26 rotates the original polarization angle 22 to a new angle 28 by means of the position of its fast axis 30. The fixed analyzer 32 then blocks all of optical signal 28 except for the particular component 32 parallel to the analyzer axis 36, which is then received by the detector 38. Detector 38 may then generate an electrical signal 40 representative of the intensity of the optical signal 34. FIG. 2 shows a reference direction 42 aligned with analyzer axis 36 of fixed analyzer 32 within an arbitrary focal plane at detector 38. The orientation of rotating half-wave plate 26 is specified by the difference angle 44 between reference direction 42 and half-wave plate axis 30. The combination of fixed analyzer 32 and rotating half-wave plate 26 can be thought of as equivalent to a single rotating analyzer that rotates twice as fast as half-wave plate 26. As shown in FIG. 2, the anti-clockwise angle 44 from reference direction 42 to half-wave plate axis 30 is doubled to give the effective analyzer position 46. Thus, by rotating half-wave plate 26 over a 180-degree range, the effective analyzer position 46 is rotated over a complete 360-degree cycle.
References to birefringence herein refer to intrinsic birefringence or form birefringence, a property of a material that causes a change in the polarization of light which passes through it. Birefringence has two components; orientation (or axis) and magnitude. Form birefringence is found in materials consisting of a substantially parallel array of many small cylindrical structures that are small with respect to the wavelength of the light passing through it. Such form birefringence is a measurable property of the retinal nerve fiber layer (RNFL) that is useful for determining RNFL thickness. Form birefringence is also a measurable property of the Henle fiber layer that is similarly usefull for determining Henle fiber layer thickness.
Knowing the thickness of a patient""s RNFL can be crucial in diagnosing glaucoma and other optic nerve diseases. The RNFL birefringence introduces retardance into any polarized beam of light passing through the RNFL when the beam polarization axis is neither parallel nor perpendicular to the nerve fiber bundles making up the RNFL. Birefringence is an optical property associated with the anisotropy of a medium through which polarized light propagates, and is manifested by the retardance of some components of the light resulting from variation of light velocity in the medium with propagation direction and polarization axis. When light propagates perpendicularly to the optic axis of an anisotropic material, the two orthogonally-polarized (S and P) components of the light, one with polarization parallel to the fast axis and the other with polarization perpendicular to the fast axis (parallel with the xe2x80x9cslowxe2x80x9d axis), travel through the material at different velocities, introducing a phase shift between the two components. This phase shift is known in the art as retardation or retardance and is herein denominated xe2x80x9cretardance.xe2x80x9d
A beam of light entering a patient""s eye encounters the retina and scatters back from it. The polarization state of the emerging directly-backscattered light changes based on the amount of retardance between the two S and P components. A retardance map can be generated based on the backscattered light that represents the thickness of the RNFL and, hence, that is useful for diagnosing maladies of the eye.
Accordingly, the commonly-assigned U.S. Pat. Nos. 5,303,709, 5,787,890, 6,112,114, and 6,137,585, entirely incorporated herein by reference, disclose laser diagnostic devices that measure the thickness of the RNFL by measuring the amount of retardance of laser light in the RNFL layer, with the amount of retardance then being correlated to RNFL thickness in accordance with principles known in the art. Likewise, the so-called Henle fiber layer, which includes photoreceptor axons and which has radially distributed slow axes centered about the fovea in the macula of the eye, is also form birefringent and consequently, its thickness also can be measured for diagnostic purposes using laser light.
However, portions of the eye (hereinafter collectively denominated xe2x80x9canterior segmentsxe2x80x9d) that are anterior to the retinal nerve and Henle fiber layers may also be birefringent. For example, both the cornea and lens are birefringent. Moreover, the axial orientation and magnitude of birefrigence of the anterior segments may vary significantly from person to person. Because a diagnostic beam must pass through these anterior segments, the laser beam retardance caused thereby must be accounted for, to isolate the retardance of posterior segments such as the retinal nerve fiber and Henle fiber layers. When measuring RNFL birefringence from the front of the eye, a compensating device is needed to remove the retardance contribution of the anterior segments from the birefringence measurement.
The above-mentioned U.S. Pat. No. 5,303,709 disclosed a corneal compensator for neutralizing the effects of the birefringence of anterior segments of the eye on a diagnostic beam meant to measure the thickness of the RNFL. The compensating structure of the ""709 patent includes a polarization-sensitive confocal system attached to a scanning laser retinal polarimeter. The detector of this apparatus includes a pinhole aperture set to be conjugate with the laser source and the posterior surface of the crystalline lens so that only reflected light from the posterior surfaces of the crystalline lens is captured and analyzed. A variable retarder is then set to null any retardance in the returned light beam, which represents a measurement of anterior segment retardance.
The above-cited ophthalmological systems send laser light traveling through the retinal nerve and Henle fiber layer structures and back, reflecting off the retinal pigment epithelium or inner retina. The light assumes a retardance (polarization bias) proportional to the amount of parallel birefringent structures (microtubules) traversed.
The commonly-assigned U.S. Pat. No. 6,356,036 B1, entirely incorporated herein by reference, discloses yet another method and apparatus for measuring the magnitude and axial orientation of birefringence in both the anterior and the posterior segments of the human eye. The anterior segment includes essentially the combined birefringence of the cornea and the crystalline lens, and the posterior segment includes regions at the fundus. The optical axis and the magnitude of the birefringence of the anterior segment is first determined, then the birefringence of the posterior segment is canceled by a variable retarder. The measured birefringence of the cornea, lens and other segments of the eye anterior to the retina are used to perform certain post-measurement calculations to provide accurate anterior segment compensation despite eye movement. The birefringence of the posterior segment is then determined without interference of the birefringence of the anterior segment. The apparatus and method are applicable to the measurement of the birefringence of the retinal nerve fiber layer at the peripapillary region and the birefringence of the Henle fiber layer at the macular region of the retina. The described a procedure uses the patient""s Henle fiber layer (instead of the lens posterior surface) as a reference surface for determining anterior segment birefringence. In principle, any useful reflecting surface in the eye can be used with the disclosed method as long as the surface may be characterized to eliminate its effects on the reflected signals.
The measuring apparatus described in the above-cited patents includes, for example, variable retarders, polarizing beam splitters and rotatable half-wave and quarter-wave retarders (xe2x80x9cwave-platesxe2x80x9d). A half-wave plate is one example of a fixed retarder or polarization rotator, which has a preferred or xe2x80x9cfastxe2x80x9d axis. Light that is linearly polarized in alignment with the fast axis passes through the fixed retarder unchanged. Light that is linearly polarized orthogonally to the fast axis is aligned with the xe2x80x9cslowxe2x80x9d axis of the retarder and is retarded in phase by an amount representing the xe2x80x9cretardancexe2x80x9d of the fixed retarder. This is, for example, one-half wavelength for a half-wave retarder, one-quarter wavelength for a quarter-wave retarder, and so forth.
As is well-known in the art, a general polarimeter may be used to measure the polarization properties, such as, for example, the Stokes Vector [I, Q, U, V], of any optical signal. The Stokes parameters form a four-component vector that completely characterizes the polarization characteristics of an optical signal. The various components of the Stokes vector may be characterized as simple combinations of intensity outputs from various combinations of linear or circular polarizers, where I is the total optical signal intensity, Q is the intensity difference between the horizontal and vertical linearly-polarized optical signal components, U is the intensity difference between the linearly-polarized optical signal components oriented at xc2x145 degrees, and V is the intensity difference between the right and left circularly-polarized optical signal components.
Disadvantageously, the retardance of a xe2x80x9chalf-wave retarderxe2x80x9d is precisely equal to one-half wavelength only at a single, optical frequency. As is well-known in the art, the accuracy of polarimetry measurements depend in part on the precision of such optical components, which may be precisely matched to a single optical wavelength, xcex0. For example, when used with light having a different wavelength, xcex, a half-wave retarder introduces a xcex/2xcex0 delay instead of a half-wave delay into the signal. Similarly, a quarter-wave retarder introduces a xcex/2xcex0 delay instead of a quarter-wave delay. While the optical frequency can be controlled very precisely, a mere 2.5 nanometer fabrication error in a half-wave retarder results in a retardance error varying from nothing to more than one degree, depending on the orientation axis of the birefringent structure being measured. Moreover, each of the optical system elements may introduce similar biasing errors, contributing to a residual system birefringence in the diagnostic optical path, which may contribute to a significant measurement error.
This is a significant problem when using the above-described polarimetry techniques to map the birefringence of the retinal nerve or Henle fiber layers in the eye because these layers have birefringence orientation axes at all angles. The system birefringence (which herein is defined to include polarization rotator retardance error) of the optical components introduces retardance measurement errors that vary unevenly over a typical RNFL or Henle fiber layer thickness map, unpredictably distorting the desired medical analysis. To keep these errors acceptably small, the (residual) system retardance over the entire diagnostic beam path must be held under two degrees. This increases the precision and care needed during the manufacture and assembly of a commercial ophthalmological apparatus for mapping the RNFL birefringence in the eye, thereby disadvantageously increasing cost and reducing measurement reliability.
There is accordingly a clearly-felt need in the art for a method that eliminates system birefringence error in the diagnostic path of a polarimeter, which would improve manufacturability and measurement reliability by relaxing tolerances during manufacture, thereby permitting the use of a wider range of components while maintaining system accuracy. The resulting improvement in polarimeter accuracy would also improve the accuracy of the above-described techniques for anterior segment retardance compensation in an ophthalmological polarimeter, thereby improving ophthalmological structure mapping accuracy. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.
This invention solves the above-described system birefringence problems by, for the first time, introducing a method for averaging multiple retardance measurement samples to cancel the effects of system birefringence in the diagnostic path. This invention results in part from the unexpectedly advantageous observation that the retardance measurement errors arising from system birefringence have a symmetry that repeats over each complete rotation cycle of optical signal polarization when effected with any useful polarization rotator, such as, for example, a half-wave retarder. Because the optical signal polarization angle rotation is doubled by the physical rotation of a half-wave retarder, the system birefringence errors repeat with every half-cycle (180 degrees) of half-wave retarder rotation. The character of this error symmetry is such that averaging the four retardance measurements collected over one such rotation cycle cancels the effects of system birefringence, leaving a mean retardance measurement free of such errors.
It is a purpose of this invention to provide an ophthalmological system and method for measuring the birefringence of structural elements in the eye with improved accuracy and eased manufacturing tolerances.
It is an advantage of this invention that combining four retardance samples over a single polarization rotation cycle cancels errors arising from system birefringence in the optical path and from any polarization rotator mismatch with the optical signal frequency, thereby reducing requisite manufacturing tolerances for the important system optical components, such as, for example, the beam splitters, lenses, scanners and retarders.
It is another advantage of this invention that the same sampling and averaging technique substantially improves accuracy and manufacturability in a general polarimeter system for measuring the polarization of any analyzed optical signal.
In one aspect, the invention is a method for analyzing a structure in the interior of an eye having a pupil, including the steps of (a) producing an optical diagnostic signal having a predetermined polarization state, (b) directing the optical diagnostic signal into the eye through the pupil, such that the optical diagnostic signal is reflected from the structure back through the pupil, (c) producing an electrical signal having a magnitude S representing the polarization state of the. reflected optical diagnostic signal as biased by a system birefringence, (d) rotating the reflected optical diagnostic signal polarization about an optical beam axis over a substantially ninety (90) degree range within which the electrical signal magnitude S varies between two extrema [Smax, Smin], (e) averaging a plurality of electrical signal magnitude extrema {Smax, Smin} obtained during rotation of the reflected optical diagnostic signal polarization over a substantially three-hundred-and-sixty (360) degree range to produce one or more mean electrical signal magnitude extrema signals [{overscore (S)}max,{overscore (S)}min] representing the polarization state of the reflected optical diagnostic signal unbiased by the system birefringence, and (f) producing, responsive to the mean electrical signal magnitude extrema signals [{overscore (S)}max,{overscore (S)}min], an analysis signal representative of a property of the structure.
In another embodiment, the invention is an apparatus for analyzing a structure in the interior of an eye having a pupil, including an optical source for producing an optical diagnostic signal having a predetermined polarization state, an optics system coupled to the optical source for directing the optical diagnostic signal into the eye through the pupil, such that the optical diagnostic signal is reflected from the structure back through the pupil to the optics system, an optical polarization detector for producing an electrical signal having a magnitude S representing the polarization state of the reflected optical diagnostic signal as biased by a system birefringence, a polarization rotator for rotating the reflected optical diagnostic signal polarization about an optical beam axis over a substantially ninety (90) degree range within which the electrical signal magnitude S varies between two extrema [Smax, Smin], a processor coupled to the optical polarization detector for producing, responsive to the polarization state of the reflected optical diagnostic signal, an image signal representative of a property of the structure, and an averager for averaging a plurality of electrical signal magnitude extrema {Smax, Smin} obtained during rotation of the reflected optical diagnostic signal polarization over a substantially three-hundred-and-sixty (360) degree range to produce one or more mean electrical signal magnitude extrema signals [{overscore (S)}max,{overscore (S)}min], representing the polarization state of the reflected optical diagnostic signal unbiased by the system birefringence.
In yet another aspect, the invention is a method for measuring the unbiased polarization state of an analyzed optical signal in an optical polarimeter system including the steps of (a) producing an electrical signal having a magnitude S representing the polarization state of the analyzed optical signal as biased by the system birefringence, (b) rotating the analyzed optical signal polarization about an optical beam axis over a substantially ninety (90) degree range within which the electrical signal magnitude S varies between two extrema [Smax, Smin], and (c) averaging a plurality of electrical signal magnitude extrema {Smax, Smin} obtained during rotation of the analyzed optical signal polarization over a substantially three-hundred-and-sixty (360) degree range to produce one or more mean electrical signal magnitude extrema signals [{overscore (S)}max,{overscore (S)}min] representing the polarization state of the analyzed optical signal unbiased by the system birefringence.
In yet another embodiment, the invention is an optical polarimeter system for measuring the polarization state of an analyzed optical signal including an input for accepting the analyzed optical signal, an optical polarization detector for producing an electrical signal having a magnitude S representing the polarization state of the analyzed optical signal as biased by a system birefringence, a polarization rotator for rotating the analyzed optical signal polarization about an optical beam axis over a substantially ninety (90) degree range within which the electrical signal magnitude S varies between two extrema [Smax, Smin], and a processor coupled to the optical polarization detector for averaging a plurality of electrical signal magnitude extrema {Smax, Smin} obtained during rotation of the analyzed optical signal polarization over a substantially three-hundred-and-sixty (360) degree range to produce one or more mean electrical signal magnitude extrema signals [{overscore (S)}max,{overscore (S)}min] representing the polarization state of the analyzed optical signal unbiased by the system birefringence.
The foregoing, together with other objects, features and advantages of this invention, can be better appreciated. with reference to the following specification, claims and the accompanying drawing.